Direct current conduction mechanism in the methyl acrylate–vinyl acetate composite thin films

Plasma polymerized (PP) methyl acrylate (MA) and vinyl acetate (VA) composite thin films were deposited onto glass substrate varying MA and VA monomer concentrations. Thickness of the composite polymers is observed to vary on the MA and VA monomer ratios, where MA is found more reactive. The FESEM images of the composite polymers show better surface morphology compared to those of the homopolymers. Appearance of broad absorption bands in the FTIR spectra of polymer indicates the structural changes compared to monomer during polymerization. Thermogravimetric analysis and differential scanning calorimetry indicate that composite films are thermally more stable (up to 617 K) compared to homopolymer thin films (563 K). The current density versus voltage (J–V) characteristics of PP(MA-VA) composite films (sandwiched between aluminum electrodes) with different MA and VA ratios showed that the J values of the composite films gradually increase with elevating VA monomer and also with temperature (298–373 K). On the other hand, this value increases with decreasing the thickness of the composite films, which complies with the other studies. The conduction of the thickness-dependent composite films showed Ohmic in nature in the lower voltage region (< 10 V) while the space charge-limited conduction is found to be dominated in the higher voltage region (> 10 V) operating over the entire range of temperature. The activation energy at room temperature was found to be ~ 0.019 eV in the Ohmic region and 0.260 eV in the non-Ohmic region.

using different techniques.Islam et al. 16 synthesized methyl methacrylate and vinyl acetate composite polymers by suspension polymerization method and reported that the resulting polymer developed chemical and physical properties compared to homopolymers made from only one of the monomers.The composite polymer was found to be thermally more stable than the homopolymers.PP Composite thin films also reported to have significantly enhanced ionic conductivity with the addition of filler 17 .
A recent publication 18 reported that poly(pyrrole-co-aniline) composite polymers made of atmospheric pressure plasma polymerization showed higher electrical resistance with a higher deposition rate at a suitable pyrrole ratio (pyrrole at a ratio of 75% to aniline at 25%).The surface morphology and the composite film thickness were also observed to be dependent on the variation of the two monomer mixture ratios because of the rapid rearrangement or incorporation of the monomer units.Additionally, Akter et al. 19 reported that the electrical conductivity of the PPTMA thin film can be adjusted by controlling the iodine doping concentration on the pure PPTMA film using the plasma polymerization method.The iodine-doped films showed two types of electrical behavior depending on the applied voltage and in the higher voltage the films obey the space charge limited conduction (SCLC) mechanism.Therefore, study of electrical properties of pp thin films could be a good choice of research.
In this study, methyl acrylate (MA) and vinyl acetate (VA) have been elected to prepare the composite films.Because the MA can be copolymerized easily with other monomer molecules to form various copolymers.The produced poly(methyl acrylate) and its copolymers have a wide range of electrical applications owing to their excellent dielectric properties and mechanical strength.VA is chosen as the other monomer because it is widely used in the production of polyvinyl acetate (PVA) and its copolymers.Both of the monomers have excellent dielectric properties and were used in electrical insulation materials, solder resists and conformal coatings to protect the conductive traces on the printed circuit board from oxidation 16,20,21 .Moreover, having a particular electron-donating acetoxy group (-OOCCH 3 ) group associated with the vinyl group, VA is considered a lessactivated monomer with a lower deposition rate 22 .When a composite is prepared from such two monomers (one more active and the other less active) their optical and structural properties are observed to vary depending on the monomer ratio and thus the electrical properties may also vary.To recognize the electrical conduction processes in the composite films charge generation and transfer in the interfaces and bulk of the films are important to be studied.Several researchers studied the current density versus voltage (J-V) characteristic in organic thin films equipped with different methods 19,[23][24][25] .The proper choice of materials include smoothness, porosity level, thermal stability, high voltage capability, and conduction.Deviation of these properties with temperature are critical factors in ensuring that the films could be efficiently utilized in electrical and electronic applications.However, the study on composite films to improve the material system and to understand how conduction operates in composite thin films is very rare.Thus, it is crucial to gain a thorough insight into the carrier transport mechanisms and how it function in the PP(MA-VA) composite thin film along with other properties.
In this work, electrical and thermal properties of the PPMA and PPVA homopolymer and PP(MA-VA) composite polymer thin films with surface morphological and structural properties were methodically examined using different ratios of MA and VA.The thermal stability and temperature-dependent DC electrical properties besides the conduction mechanism of the thickness-dependent composite films have been studied to explore the potential compatibility of the thin films for electrical and electronic devices of modified required applications.

Materials and methods
The monomers MA and VA monomer precursors used for preparing the PP composite thin films are obtained from BDH, Poole, England and Merck, Germany, respectively.The chemical formula, boiling point and density of MA are CH 2 CHCO 2 CH 3 , 353 K and 950 kg/m 3 and those of VA are CH 3 CO 2 CHCH 2 , 345.7 K and 934 kg/m 3 , respectively.However, the molecular weight of both monomers is same (86.09g/mol) and the chemical structures of the monomers are shown in Fig. 1 Experimental details The pre-cleaned glass slide (25.4 × 76.2 × 1.2 mm 3 , Sail brand, China) are taken to deposit PP thin films.A detailed explanation of the plasma polymerization process is described in earlier research 26 .The plasma reactor consists of two electrodes made of stainless steel that are placed in close proximity to each other to create a uniform electric field.Before starting deposition the plasma chamber is vauumed to a low pressure of 0.1 Torr using a rotary pump.To ignight discharge plasma glow the electrodes are connected to an AC (50 Hz, 22 W) power  www.nature.com/scientificreports/supply and the films are deposited for 60 min.The vapor of the composite monomer solution entered into the plasma chamber due to highly pressure difference between the monomer container and the chamber.The flow rate is kept constant at ~ 20 cm 3 /min with the help of an injecting valve.The films are deposited by fluctuating the amount of MA (100%, 75%, 50%, 25% and 0%) in the mixture of MA and VA monomer and the films are defined as PPMA, PP(MA-VA) (3:1), PP(MA-VA) (1:1), PP(MA-VA) (1:3) and PPVA respectively.The as-deposited film thicknesses, d, are measured by multiple-beam interferometric technique 27 using the following equation: where λ (= 589.3 nm) is the wavelength of the light source (sodium) and b a is the fractional discontinuity in the produced fringe interfaces as shown in Fig. 2. Before deposition half of the substrate was covered using Teflon taps and the remaing uncovered side was allotted to deposit the films.The film's thickness was measured at multiple points, and the average thickness was calculated based on at least 8 measurements.All of the films are deposited in the same plasma conditions (for 60 min) but the thicknesses of the resulting films are found to be 213 ± 7, 197 ± 4, 175 ± 4, 137 ± 5 and 117 ± 4 nm for PPMA, PP(MA-VA) (3:1), PP(MA-VA) (1:1), PP(MA-VA) (1:3) and PPVA, respectively.This is because MA is more reactive monomer compared to VA.Although both MA and VA monomers contain a vinyl group (CH 2 =CH-), there are certain structural differences that can influence their reactivity.MA has an ester group (-COOCH 3 ) adjacent to the vinyl group and the presence of the ester group introduces an electron-withdrawing carbonyl functionality 22 .This increased reactivity of MA lead to faster and more efficient polymerization reactions to form plasma polymerized films during the polymerization process.In contrast, VA contains an additional electron-donating acetoxy group (-OOCCH 3 ) and this nature can stabilize the electron density around the vinyl group, making it less reactive toward the nucleophilic attraction.As a result more deposition rate of MA increase the film's thickness even the plasma is operated for the same duration.

Characterization techniques
Surface morphology of the as-deposited thin films is examined at an accelerating voltage of 5 kV using field emission scanning electron microscope (FESEM) (JEOL JSM 7600F, USA).The deposited films are masked with platinum to evade charging effects throughout the measurement.
The associated functional groups of the films are observed by a double-beam Fourier transform infrared spectroscopy (FTIR) (SIMADZU, FTIR-8400) with a resolution of 2 cm −1 to elucidate the chemical structure of the homo polymer and composite polymer thin films.
To observe thermal stability, thermogravimetric (TG) and differential scanning calorimetry (DSC) are done under N 2 atmosphere using a computer-controlled TG/DSC thermal analyzer (Model: NETZSCH STA 449 F3 Jupiter).The heating rate was kept at 10 K/min and the analyzer employs a horizontal system balance mechanism.The powder samples for FTIR spectroscopy are gathered from the substrate's surface.

DC electrical measurement
Pure (99.99%) aluminum (Al) (Willacy, Japan) is used for electroding the samples, and Al/PP(MA-VA)/Al sandwich-type structure is prepared for electrical measurements as shown in Fig. 3. Firstly, the lower Al electrode

Analysis of surface morphology
Figure 4 depicts the FESEM images of homo polymer and composite polymer thin films captured at a magnification of 50 k.The PP(MA-VA)(1:1) composite film is observed to be more smooth, scratch-less, and pinholefree compared to the PPMA and PPVA films surface.No visible grain or cluster is found on the composite film surfaces, while some cluster or mosaic-type structures are visible on the homo polymer surfaces.This type of mosaic-like surface was also found for homopolymers in some other studies deposited by plasma polymerization technique 28,29 .It is speculated that the conformation of two distinct monomers boosted the crosslinking among the plasma radicals, ions, or molecules by the collisions of energetic electrons, which reduces the polymer chain and resulted in improved surface morphology of the composite films 30 .A details description of surface morphology is reported in our earlier research work 31 .

FTIR analysis
The FTIR spectra of the homo polymer and composite polymer films were described in detail in our recent publication 31 .The presence of absorption peaks related to the stretching vibration of methyl group (-CH 3 ) and the bending vibration of C-H at 2922-2970 cm −1 and 1378 cm −132 , respectively, confirmed the organic nature of the compounds.In the fingerprint region of the MA and VA monomers, some sharp absorption peaks appear but all the peaks are not present or merged in the spectra of the deposited films.This indicates the reorganization or recombination of the monomer molecules and formed highly crosslinking PP thin films 32 .

Thermal analysis
The TG curves of PPMA, PPVA, and PP(MA-VA) powder samples taken under N 2 environment are shown in Fig. 5a and provide valuable information about thermal stability of the thin films.TG is a technique that tracks the percentage of weight variation of a sample during a heating process, and the resulting curves provide information on the thermal degradation of the samples.In this case, the TG curves show two stages of degradation, A and B, with the gradual increase in temperature.At temperatures between 303 to 642 K (region A) minimal weight loss (1.2-1.4%) is observed, which may owing to the loss of low molecular mass species like H 2 O and CO 2 , which are generated during the thermal stabilization process 33 .This low percentage of weight loss in these regions also indicates the hydrophobic behavior of the as-deposited PP thin films.On the other hand, region B shows significant weight loss due to the thermal decomposition or breakdown in the polymer chain.The percentage of thermal decomposition is 7% and 10% for PPVA and PPMA homopolymers, respectively, and approximately 5%, 6%, and 7% are for the PP(MA-VA) (1:3), PP(MA-VA)(1:1) and PP(MA-VA)(3:1) composite polymer films, respectively.However, the weight loss decreases with the increased amount of VA in the PP(MA-VA) composite films.This is due to the development of stronger crosslinking during the formation of composite polymeric chains, which increases the thermal stability of the films and helps the films to resist the molecules being thermally degraded 34 .
The DSC melting curves of homo polymers and composite polymers as shown in Fig. 5b, exhibit significant degradation of exothermic peaks.The maximum stability temperatures of PPMA and PPVA are 563 K and 571 K, respectively.Beyond this temperature, an endothermic reaction is initiated due to the thermal breakdown of the polymering chains.However, for the composite polymer samples of PP(MA-VA) (1:3), PP(MA-VA)(1:1) and PP(MA-VA)(3:1), the exothermic peaks are observed at around 617, 585, and 581 K, respectively.As the VA monomer concentration increases, the exothermic band peaks shift towards a higher temperature range.This suggests that PP(MA-VA) composite films are more thermally stable than any of the homopolymers due to the copolymerization effect.

The current density versus voltage characteristics
The J-V characteristics of PPMA and PPVA homo polymer and PP(MA-VA) composite polymer thin films are recorded at various temperatures within the voltage range of 0.3 V to 100 V and the temperature range of 298 K to 373 K. Figure 6 indicates a change in the current density in the same voltage region for all homo polymer and composite polymer thin films.Moreover, it can be observed that J increases as the proportion of VA monomer in the composite structures increases.This may be due to decline the thickness of the composite film with VA monomer, resulting in more electron transfers from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) level and the creation of traps that enhance the electrical conductivity 35 .In polymers, these traps are considered as extrinsic charge carriers.The J-V characteristics of the homo polymer and composite polymer films obey a power law of J ∝ V n , where n is the power index, and the slopes of n ≤ 1 indicate an approximately Ohmic type conduction, while 1 < n represents a non-Ohmic conduction at higher voltages 23,36 .The n value of all the as-deposited films are found to lie in the range of 0.15 to 0.98 in the low voltage region (≤ 10 V) and 1.00 to 2.65 in the high voltage region (> 10 V), respectively indicating Ohmic conduction and non-Ohmic conduction mechanism, respectively.

Thickness-dependent conduction mechanism
To determine the direct current (DC) conduction mechanism in the thickness-dependent composite thin films, the PP(MA-VA)(1:1) is deposited for 30, 45, 60, 75, and 90 min and the thicknesses of the films are observed to be 117 ± 5, 139 ± 6, 151 ± 5, 165 ± 4 and 181 ± 6 nm, respectively.The J-V characteristics curves of these films are shown in Fig. 7, and it is observed that the J increases as the film thickness decreases within the same applied voltage.This may be attributed to a decrease in the number of grain boundaries as the film thickness decreases, which in turn reduces the barriers to charge transport and increases conductivity.Additionally, a decrease in film thickness reduces the distance between the electrodes leading to an increase in electric field strength, which also contributes to an increase in current density 37 .These findings are consistent with some previous reports 23,38 .
In the lower voltage region (< 10 V), the value of n is observed between 0.16 and 0.98 for the thickness dependent samples again indicating an Ohmic-type conduction mechanism.At higher voltages, the n value is between 1.00 to 2.65, suggesting a non-Ohmic conduction mechanism, which may be space charge limited (SCLC), Schottky, or Poole-Frenkel (PF) 36,39 .The dependence of J on the distance between the electrodes, that is the thickness of the films (d) follows the power law J ∝ d −l , where l is a parameter that depends on the presence of traps in the films.When |l|≥ 3 the mechanism would be SCLC and when |l|< 3 the mechanism would be Schottky or PF 36,40 .Therefore, to investigate the conduction mechanism in the PP(MA-VA) films, J is plotted against d as shown in Fig. 8 for the thicknesses dependent films in the non-Ohmic region at three different voltages of 50, 65 and 80 V.In all the cases the negative slopes are 5.89, 5.31 and 5.60, respectively, indicating that neither Schottky nor PF mechanisms are present in these films.The main conduction mechanism in these films is SCLC.In SCLC process, when an external voltage is applied across the film, charge carriers (electrons or holes) are introduced from electrode to the polymer.These carriers then interact with traps, which are defects or localized states with energy variations within the band gap of the material.In these circumstances, the carriers might get stuck into these traps or move through the material maintaining dynamic equilibrium 41,42 .With an increase in voltage, more carriers are injected, causing a buildup of charges close to the injection electrode.This accumulation generates an internal electric field that counteracts further carrier injection, leading to the distinctive non-Ohmic behavior characteristic of SCLC 41,42 .

Dependence of J on temperature
In a polymer system, carrier mobility is usually deficient.Temperature, concentration of conducting species, dopant level, polymer shape and other parameters can affect the conductivity.With temperature, the charge carrier density grows sharply, which indicates the temperature dependence on conductivity, but poor field conduction results in an exponential temperature dependency.The Arrhenius equation (Eq.2) is commonly used to analyze the effect of temperature on current density 43 .
Where, k B = Boltzmann constant, ΔE = Activation energy and J 0 = Current density at thermal equilibrium.The dependence of J on 1/T for thickness-dependent composite films is shown in Fig. 9.The curves indicate that the conductivity is elevated with higher temperature in both Ohmic and non-Ohmic regions, which leads to the higher ΔE with temperature.The values of ΔE are almost 0.019 eV in the Ohmic region (at 7 V), and 0.260 eV in the non-Ohmic region as presented in Table 1.These changes in ΔE are related to the structural changes of the polymer with temperature.
In polymer, the charge carrier moves by transferring the thermally triggered charge carriers among the localized states over a potential barrier, which is known as hopping conduction 44 .To overcome the potential barrier, those charge carriers obtain adequate energy from the lattice through thermal oscillations.The reduction in ΔE with declining temperature indicates a successive transition to the hopping regime.Therefore it is inferred that trap-mediated SCLC mechanism is observed in this thickness-dependent composite thin films.

Conclusions
A low-pressure capacitively coupled glow discharge reactor is used to successfully deposit desired PPMA, PPVA, and PP(MA-VA) films onto glass substrates.The combination of two organic monomers increased the polymerization rates and allowed a wider range of polymeric properties.The thickness of the PP(MA-VA) composite films decreased from 213 to 117 nm as the concentration of MA monomer in the mixture decreased.Composite polymer films showed improved surface morphology with smooth, homogeneous, and pinhole-free surfaces as compared to PPMA and PPVA homopolymers.During plasma polymerization the structure of the monomer changes significantly due to the reorganization of molecules.The composite films are thermally stable up to 617 K, and this stability increased with a higher concentration of VA.The electrical conductivity increased for the sample of lower thickness and at higher temperatures.The maximum value of ΔE at room temperature in the

Figure 2 .
Figure 2. Image of the fringes occupied from multiple-beam interferometric set-up.

Figure 3 .
Figure 3. (a) Photograph of the electrical sample and (b) schematic drawing with electrode assemblage of Al/ PP(MA-VA)/Al sandwich-type thin film.

Figure 7 .
Figure 7. J-V relationship of the PP(MA-VA) composite thin films of thicknesses (a) 120 nm, (b) 135 nm, and (c) 165 nm at different temperatures, (d) for the different thi cknesses at room temperature (298 K).

Figure 8 .
Figure 8. Plots of J versus d for PP(MA-VA) composite thin films in the non-Ohmic region (at 50, 65 and 80 V).

Figure 9 .
Figure 9. Plots of J versus 1/T of thickness-dependent composite thin films in the (a) Ohmic and (b) non-Ohmic regions.

Table 1 .
Values of activation energy ΔE (eV) for thickness-dependent PP(MA-VA) at Ohmic and non-Ohmic voltage regions.